How does a prop (Dash 8, for example) reverse thrust? I know It's not the same as say a 737 or an A320, but when Horizon Dash 8s land on the shorter runway at SEA or a small airport like PSC or PDT, the engines sound like they are slowing the plane down. Thank you!

Props on those types of planes as well as some small aircraft like cessnas have what's called prop feathering...This is used so that the engine speed can remain constant and the blades can be turned so that they can take a bigger bite out of the air...After landing the blades are reversed so the spinning action of the engine does not pull air over the wings but rather blows it all forward away from the plane

Actually, prop feathering refers to the prop blades being turned to parallel the airstream so zero thrust is produced. This is helpful in an engine failure to drag on the powerless prop can be minimized.

All complex airplanes and turboprops have constant speed propellors (also known as variable pitch props). The angle of the propeller blades varies to control the rpms. Not all airplanes with constant speed props also have feathering capability. This is normally only found on twins and turboprops, although not all twins can feather either. A complex airplane is any airplane that has retractable gear, flaps, and a constant speed prop.

To produce reverse thrust, the angle of the blades is changed so they push air forward instead of pulling it back, but usually only turboprops are able to do this.

It might help to visuallize that each blade on one of these large propellor assemblies is a separate unit which can be detached from the hub. As stated in the previous post, the angle at which the blade is meeting the air can be changed. This is used to increase or decrease the speed of the prop, or how much power the engine is transmitting to the prop to increase the efficiency of the engine.

Large recip engines use the engine oil pressure to provide muscle to adjust the blade angle. The Allison 501D engine (C-130) uses this system also. One benefit of this is that you can spring-load the blades to feather when there is no oil pressure, thus giving automatic protection in case of an engine failure and ease of starting.

When one of these engines is put into 'reverse', the blades are just forced past the 'feather point'. This causes the propeller to push air, not pull it.

One other way to look at this system is to think of a helicopter. If you are familiar with how a rotorhead works, it is the same concept. When the pilot pulls up on the collective lever, the angle of all the rotor blades changes, increasing the bite of air they are taking. If he could pull the collective far enough, the blades would be vertical, and a little farther would cause them to push air up rather than pull it down.

Reverse thrust occurs when the blade angle is altered to produce and negative angle of angle. The result of this is that an aerodymanic force is produced in a direction behind the propeller, resulting in thrust acting in the same direction as drag. The same thing occurs when the propeller is windmilling (the large increase in drag) except propeller torque acts in the same direction as the propeller, resulting in a possiblitiy of overspeed.

Depending on the system the blade angle can be altered in one of three ways.
the first way is to use oil pressure to both fine and coarsen the blade. (the method used on most aircraft with reserve thrust)
The second is to use oil pressure to coarsen the blade and CTM (cenfigual turning moment) the fine the blade
The thrid and commonly used method (amongst light twins) is to use oil pressure to fine the blade angle and ATM (aerodynaimc turning moment) with the assistance of a counterweight (so to counter to CTM which has a stronger tendency) to coarsen the blade. the reason why this method is perferred is because a failure of oil pressure will result in the blade feathering (ie reduced drag), as opposed to using CTM which will course the blade to fine (ie lots of drag) out with a oil pressure failure.

That's exactly what they're doing, reversing like a jet...but the props do not stop and spin the other way! Usually they use it in Sea-Tac or PDX for example to get the quickest available taxiway in those busier traffic patterns.

Jim is on the right track for most turboprops. Propeller blade angles are measured fore & aft of zero degrees. At zero degrees, with the aircraft not moving and the engine running, there is theoretically zero thrust. During this phase of operation, the propeller blade angle is controlled directly by the power lever (throttle). Moving the power lever forward places it in the ground idle detente and moves the propeller blade angle to (typically) about +11 degrees. Movement of the power lever to this point does not change engine or propeller RPM, just blade angle. Further movement forward of the power lever increases engine RPM and consequently propeller RPM. As prop RPM increases, governers within the propeller dome prevent the RPM from increasing beyond certain amounts (typical to manufacturer). The way the governor restricts prop RPM is to increase prop pitch further. As an example, during take off, the engine and propeller of a PW100 engine typically reach 90% torque and 1210 RPM. Once the 1210 RPM is reached, prop pitch is increased/decreased by the prop governor to maintain that RPM (if RPM decreases, prop governor increases oil pressure to reduce prop pitch; if RPM increases beyond 1210, the prop governor reduces oil pressure to the prop dome causing an increase in prop pitch).

Confused? It gets better. As stated earlier, with respect turboprops, oil pressure drives the propeller from coarse pitch to fine then reverse pitch. Feathering springs counter the oil pressure and try to drive the prop pitch to full coarse/feather pitch.

Remembering the description of "degrees" above, if zero degrees represents zero thrust at zero forward airspeed, then the same zero degrees of pitch will represent a great deal of drag if the airplane is moving forward at say 100 knots (as in landing). The position of the props at this zero degree setting is called "discing". Discing is frequently used after landing to help slow the airplane, and utilizes minimal fuel and produces the least amount of noise. The power levers usually have some kind of detente at the discing position.

To prevent the pilot from inadvertantly selecting "discing" while in flight, so too does the flight idle setting of the power levers have a detente. On some aircraft this can easily be overridden, even in flight, but it takes conscious effort, and can be very dangerous.

Not finished yet:

Once on the ground during rollout, if more stopping energy is desired from the powerplant, continuing to retard the power levers moves the prop blades into negative territory, i.e. -15 degrees (typical). At the same time, the power levers cause the engine to accelerate. More engine power + increasing large negative prop blade angles = increased reverse "thrust".

Most modern turboprops have very distinctive noises when taxiing on the ground. Most pilots use little or no braking while taxiing. Both these phenomena are directly related to the fine movements of the power levers while manoeuvring on the ground, with minute changes of the propeller pitch. The engine speed/power remains unchanged.

In closing, the range of pitch of the turboprop blade goes from (approximately) 90 degrees (feathered) back through the entire range to minus 15 degrees (full reverse pitch). Forward flight blade angles range from about 17 degrees to about 70 degrees. These measurements are all made with respect the prop installation on the engine and have no signifigance with "angle of attack".

Hope that clears up some of the mystery without producing more confusion!

I just wanted to say thanks for that thorough explanation. I have often wondered about the intricacies of prop movement on turbine engines. Was also nice that you took the time to explain it in a way that was understandable.